Overview of a Peer-Reviewed Scientific Report

“Absence of Evidence of Xenotropic Murine Leukemia Virus-Related Virus Infection in Persons with Chronic Fatigue Syndrome and Healthy Controls in the United States”
by William Switzer, Hongwei Jia, Oliver Hohn, et al. Retrovirology 2010, 7:57 (July 1, 2010)


On July 1, 2010, CDC researchers and colleagues from two institutions reported results of a study in which they found no evidence of infection with xenotropic murine leukemia virus-related virus (XMRV) among patients with chronic fatigue syndrome (CFS), a serious disorder that affects 1 to 4 million U.S. adults. The CDC-led team used a combination of different molecular and serologic assays to test archived blood specimens from CFS patients and healthy controls. Results of blinded testing performed at CDC and two other laboratories were negative for XMRV. These findings are consistent with three recent studies done in Europe that found no link between XMRV and CFS, but they are in sharp contrast to reports by Lombardi et al (Science 2009;326:585) and Lo et al (Proceedings of the National Academy of Sciences 2010) that found evidence of XMRV and MLVs, respectively, in large percentages of CFS patients and smaller percentages of healthy people. The CDC report, published in the open-access journal Retrovirology, is available at Absence of evidence of Xenotropic Murine Leukemia Virus-related virus infection in persons with Chronic Fatigue Syndrome and healthy controls in the United StatesExternal.

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About the CDC Study

The test paper by CDDC scientists and colleagues is the first U.S. report on XMRV and CFS since the October 2009 publication in Science that suggested a possible link between this virus and CFS. XMRV is a newly reported human retrovirus that was first identified in 2006 in tissue specimens from men with prostate cancer. CFS is a complex and debilitating condition characterized by severe fatigue lasting at least 6 consecutive months as well as other signs of illness. Its cause is unknown and no reliable diagnostic tests specific for CFS are available.

To evaluate a possible association of XMRV with CFS, the CDC-led team tested blood specimens from 51 persons with CFS and 56 healthy persons. The specimens were from previous study groups: 1) people who had taken part in population-based studies of CFS in Wichita, Kansas, and in Georgia, and 2) patients in Georgia who had been referred by physicians to a registry of fatiguing illness. Patients from both groups had CFS that met the criteria of the 1994 International CFS Research Case Definition, which was established to help distinguish CFS from other illnesses that cause fatigue.

Comprehensive molecular and serologic testing was performed independently at three laboratories blinded to the clinical status of the study participants. Detection of XMRV DNA sequences was done by using three PCR tests (two gag and one generic polymerase) at two laboratories. Detection of XMRV antibodies was done by using three different serologic assays (Western blot, ELISA, and immunofluorescence assay [IFA]) at two laboratories.

Testing of all specimens was negative for XMRV.  Laboratory test results include the following:

  • Archived plasma specimens from 51 CFS patients and 53 healthy controls from Wichita, Kansas, as well as metropolitan, urban and rural populations in Georgia were tested by a Western blot assay at CDC. All specimens tested negative for antibodies to XMRV.
  • Additional blinded screening of the same 51 CFS cases and 53 controls at an independent laboratory using an ELISA identified weak seroreactivity in one CFS case and a healthy control, but both were negative by IFA and Western blot.
  • PCR testing at CDC showed negative results for XMRV DNA from 50 CFS persons and 56 controls and 41 U.S. blood donors.
  • Blinded testing by a second PCR assay in an independent laboratory was also negative for DNA specimens from the same 50 CFS persons and 56 controls.

The authors of the Retrovirology paper concluded that their data do not support an association of XMRV with CFS.

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Do the results of the CDC study show conclusively that there is no link between XMRV and CFS?

The study found no association between XMRV and CFS in this population of CFS patients and controls. However, these results do not necessarily extend to other populations or locations. For example, the authors note that their findings “may not be generalizable beyond our study populations because XMRV infection rates may vary in different regions or locales.” Additional research will be necessary to learn more about XMRV and any association that might exist with poor health outcomes, including CFS.

What did the studies by Lombardi et al and Lo et al find?

Lombardi and colleagues (Science 2009;326:585) reported detecting XMRV in approximately two-thirds of patients diagnosed with CFS and 4 percent of healthy people. Lo and colleagues (Proceedings of the National Academy of Sciences 2010) reported finding MLV-like viral gene sequences in about 87 percent of CFS patients and 7percent of blood donors. Both research teams concluded that their findings support an association between the retroviruses they detected and CFS, but noted the results do not prove that these viruses can cause CFS. Both teams also raised questions about a potential risk of these viruses to the blood supply. These findings are in contrast to those of the CDC team, which found no evidience of XMRV infection in their study populations of CFS patients and healthy controls.

Are the different findings due to differences in the research tests to detect XMRV and MLVs?

No. An independent assessment of the laboratory methods used by CDC and FDA showed that both laboratories were able to detect XMRV at low levels in blinded samples. In addition, CDC’s laboratory provided samples from its study to FDA for testing, and initial analysis shows the FDA results were generally consistent with those of CDC.

What other reasons might explain the differences in results?

It is not clear why the findings of the reports differ. Possible explanations include selection criteria for inclusion of CFS patients, differing clinical subsets of CFS, and possible variations in XMRV and MLV infection rates among populations in different geographic regions. There also may be other, as-yet-unknown factors that could help explain the different results of the studies.  Scientists involved in these studies are working collaboratively to design experiments to answer these scientifically puzzling questions.    

One important consideration is that XMRV is a newly identified virus, first reported in 2006, and much remains to be learned about this and related viruses, such as MLVs.  As additional research is done on XMRV and similar viruses, it is possible that new findings might emerge that differ from those reported to date.

What populations of CFS patients were evaluated in the CDC study and in the other two studies?

The CDC-led team tested specimens collected from previous population-based studies in Kansas and in Georgia and from physician referrals in Georgia. All CFS patients received a clinical assessment and had a diagnosis that met the criteria of the 1994 International CFS Research Case Definition. The use of specimens from two types of CFS study populations (population-based and physician referred) enabled the CDC study team to assess potentially different types of CFS illness. For example, the participants from population-based studies tend to have a more gradual onset of illness compared with physician-referred patients, who tend to have a more sudden onset of symptoms.

The study reported in Science tested samples from a repository containing specimens from CFS patients.

The FDA/NIH study tested blood specimens collected from the New England area in the mid-1990s from 37 patients diagnosed with CFS, as well as samples from 44 healthy blood donors collected by NIH from 2003 to 2006. Follow-up samples were collected from eight of the CFS patients in 2010, and seven of these were again positive for MLV-like gene sequences.

What do these recent findings from CDC and FDA/NIH mean for CFS patients and clinicians who treat them?

Further studies are necessary to determine if XMRV or other MLV-like viruses are reproducibly associated with CFS, and if so whether these viruses are causative agents. The different findings from various studies reinforce the need for more research, including a careful analysis of other populations of CFS patients from different geographic regions, and studies of larger populations of healthy people.  CDC, FDA, and NIH investigators have been collaborating with scientists from other agencies and groups involved in this research and will continue to do so.

What do the findings mean for CDC’s future work in CFS research?

CDC’s CFS program will continue to carry out a comprehensive research agenda designed to examine a wide range of factors that could provide greater insight into the causes of and best treatments for CFS. The direction of future research will be guided by results and insights gained from ongoing scientific investigations.

Are additional studies on XMRV planned that might help clarify differences reported by various research groups?

Yes. One important next step is to establish testing methods that can be used consistently across XMRV and MLV studies. To pursue this objective, CDC, FDA, NIH and several non-federal laboratories are participating in an XMRV assay comparability study, which is being coordinated by a working group of the U.S. Department of Health and Human Services (HHS). When completed, this is expected tohelp researchers to more precisely replicate past studies and further study a potential link between XMRV and MLV and adverse health outcomes, including CFS.

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What is already known about this subject?

Chronic fatigue syndrome is a complex disorder characterized by severe fatigue lasting at least 6 months and other health problems; it affects 1-4 million U.S. adults, its cause is unknown, and it is difficult to diagnose. XMRV is a newly identified human retrovirus that a previous study found to be associated with CFS.  A more recent study by FDA/NIH researchers found murine leukemia viruses (MLVs), which are closely related to XMRV, to be associated with CFS. Both studies also raised the possibility that XMRV/MLVs might be spread by blood transfusion.

What is added by this CDC report?

CDC scientists and colleagues, using a combination of molecular and serologic tests, found no evidence of XMRV in people with CFS or in healthy people.

What are the implications for public health practice?

Additional studies are needed to further evaluate if XMRV and MLVs are associated with adverse health outcomes, including CFS.